1. Field of the Invention
The present invention relates to measuring moisture content in moist environments by using the a.c. conductivity of a strip of hygroscopic polymer Nafion. The conductivity is a function of humidity and temperature.
2. Description of Prior Art
A long-lasting electrolytic hygrometer capable of use in any moist gaseous environment, based on a measurement of d.c. current required to electrolyze water adsorbed by Nafion, was described by D. D. Lawson (ref. 1) in U.S. Pat. No. 4,083,765. The configuration of this sensor was tubular; moist air was drawn in through a Nafion tube and electrolyzed to a totally dry state. From the flow rate and current, humidity was calculated. This type of hygrometer measures the humidity directly in terms of mass flow rate and electric current and is therefore linear in humidity. Also, temperature has no influence on the moisture content. Although these features are desirable, it is considered that a simpler scheme using only one conductivity measurement is advantageous in a harsh environments. This is discussed later.
Moisture measurements are necessary for the control of many industrial processes where drying is involved. For example, control of drying in a paper mill requires a trouble-free, rugged sensor able to withstand moderate temperatures (&lt;80.degree. C.), atmospheres with organic vapors and dust and air speeds of the order of 10 m/s. Similar harsh environments occur in other industries. Many hygroscopic materials which have been used as sensing elements for hygrometers eventually become unreliable in operation because of surface contamination.
Polymers that adsorb water have been used as humidity sensors using a measurement of capacitance to infer moisture content. This type is made by Rotronic Instruments Corporation (ref. 2).
Pacer Systems offers a moisture sensor based on the adsorption of ultraviolet light by water vapor which is claimed to withstand 250.degree. C. (ref. 3). Keeping the optical windows clean in dusty atmospheres may be difficult with sensors of this type.
Dew point hygrometers are based on thermoelectric cooling of a surface till dew settles on it. This condition is detected by a change of light reflection, capacitance (ref. 4) or attenuation of surface acoustic waves (ref. 5). Such sensors are not immune to surface contamination. In harsh environments, they perform very poorly.
The patent "Humidity Sensing and Measurement Employing Halogenated Organic Polymer Membranes," by Huang (ref. 6, U.S. Pat. No. 4,681,855) claims that a polymer like Nafion but with acid groups of both strong and weak acids (sulfonic acid and carboxylic acid in particular) has superior linearity and hysteresis properties. The claim of linearity is erroneous because Huang's supporting data clearly show that the conductance is a straight line in a logarithmic plot when plotted vs. relative humidity, which represents a power law. For example, this material changes its conductivity by a factor of 4000 when humidity changes from 30 to 100%!
Bare Nafion is much better because the relation between conductance and humidity is only a third degree curve over the whole range from 0 to 100%. This polymer Nafion is clearly usable over the whole range of humidities and not only upto 40% as erroneously claimed by Huang. The polymer containing two acid groups is inferior to bare Nafion as a humidity sensing element.
It turns out that lithium doped Nafion sensors are really linear over the range of humidities from 40% to 95%, in the range of temperatures of interest in paper drying application. Both Nafion and lithium-doped Nafion have excellent stability properties as well as negligible hysteresis. The present disclosure explains these in detail.